1. Field of the Invention
The invention relates to a novel process for preparing cyclic alcohols by oxidation of cycloalkanes having from 9 to 16 carbon atoms using oxygencontaining gases In the presence of boric acid.
2. Discussion of the Background
Cyclic alcohols having large rings are valuable intermediates in the preparation of odorants, pharmaceuticals, agrochemicals and for preparing precursors of polymers.
The oxidation of paraffins by oxygen in the presence of boric acid has long been known and is described, for example, in DRP 552 886 (1928). This oxidation predominantly halts at the alcohol stage, which is explained by the boric acid trapping the alcohol by ester formation and thus removing it from further oxidation to the ketone, and if appropriate to the carboxylic acid.
According to H. Kalenda, Ullmanns Encyklopxc3xa4die der technischen Chemie (Ullmann""s Encyclopedia of Industrial Chemistry), Volume 9, 1975, p. 674, cyclododecanol is prepared by oxidation of cyclododecane with boric acid, cyclododecanone, inter alia, also being formed as a secondary oxidation product. The work of F. Broich and H. Grasemann, Erdxc3x6l-Kohle-Erdgas-Petrochemie 18, 1965, pp. 360-364, is concerned with the reaction mechanism of the air oxidation of cyclic hydrocarbons. The oxidation of cyclododecane in the presence of boric acid is described here also.
Ullmanns Encyklopxc3xa4die der technischen Chemie (Ullmann""s Encyclopedia of Industrial Chemistry), Supplementary Volume, 1970, pp. 170-177, in the discussion of the oxidation of saturated hydrocarbons by air, also describes the oxidation of cyclohexane to cyclohexanol and cyclohexanone. Oxidation in the presence of not only cobalt, but also boric acid is described here. However, in view of the higher energy and capital costs of the boric acid process, the cobalt process has established itself in the oxidation of cyclohexane.
In the oxidation of cyclododecane to cyclododecanol and cyclododecanone, in contrast, only boric acid is employed, because this has achieved yields of cyclododecanol/cyclodecanone-mixtures of 75 to 80%, even in the conversion range of 25 to 30%. In contrast, the oxidation of small rings is carried out using cobalt catalysts. Thus, for example, U.S. Pat. No. 4,263,453 discloses that cyclohexane can be oxidized to adipic acid using, for example, cobalt acetate.
According to EP-A-0 519 569, preferably, cycloalkanes having from 3 to 8 carbon atoms are oxidized on a molecular sieve comprising cobalt(II) ions. Cyclohexane predominantly produces cyclohexanol, cyclohexanone and adipic acid.
In the oxidation of cyclohexanol to cyclohexanol and cyclohexanone according to U.S. Pat. No. 5,767,320, a solid phthalocyanine or porphyrin complex of a transition metal is used as a catalyst. In this publication, cobalt complexes are also used in the examples. However, the process is restricted to cyclohexane and is carried out without boric acid.
An object of the present invention is to increase the reaction rate of the oxidation of large rings having from 9 to 16 carbon atoms to cyclic alcohols with boric acid, without impairing the selectivity of the reaction.
The object is achieved charging the cycloalkane and from 0.2 to 5% by weight of boric acid, based on the cycloalkane, adding during the reaction sufficient boric acid so that the molar ratio of cyclic alcohol formed to the boric acid at the end of the reaction is from 1:0.6 to 1:1.7 and, furthermore, performing the reaction in the presence of from 0.05 to 5% by weight of cobalt(II), based on the cycloalkane originally charged. Surprisingly, the feeding of boric acid and the addition of cobalt(II) lead to a significant increase in the oxidation reaction rate.
In the oxidation reaction, preferably, a molar ratio of cyclic alcohol formed to boric acid of from 1:0.8 to 1:1.4 is established, roughly equimolar amounts being very particularly favorable. During the reaction, the boric acid is preferably added in from 1 to 10 portions. Boric acid may also be added continuously.
For the oxidation, use is preferably made of xcex1-metaboric acid or a boric acid which forms xcex1-metaboric acid, for example, such as orthoboric acid, which dehydrates to form xcex1-metaboric acid at the oxidation temperatures.
The concentration of cobalt(II) is preferably from 0.1 to 2% by weight, based on the amount of cycloalkane at the start of the reaction. This is based on the cobalt ion and not the cobalt salt. Cobalt(II) is generally used as an organic or inorganic salt. Preferably, salts or carboxylic acids having from 2 to 18 carbon atoms are used. Examples of these are acetate, oxalate, dodecanoate, palmitate and stearate, and mixtures thereof.
The oxidizing gases preferably comprise from 10 to 100% oxygen, air being very particularly preferred as an oxidizing agent for cost and safety reasons.
Examples of large rings are cyclononane, cyclodecane, cyclododecane, cyclotridecane and cyclohexadecane. Preferably, cycloalkanes having from 10 to 14 carbon atoms are used.
The oxidation process essentially yields a reaction mixture of cycloalkanol boric ester, cycloalkanone and cycloalkane. In a further process step, the reaction product may be subjected to hydrolysis with water at elevated temperatures. The resulting product breaks down on cooling into 2 phases, an organic phase having the cycloalkanol, the cycloalkanone and the cycloalkane, and an aqueous phase having the boric acid and the cobalt. The organic phase is cobalt-free. Boric acid can be separated off from the aqueous phase by crystallization, whereupon boric acid and cobalt can be recycled to the oxidation process.
When referring to the presence of, or an amount of, a cycloalkanol or boric acid, cycloalkanol boric acid esters are included, with the weight or molar amount corresponding to weight or molar amount of the respective compound which would result after complete hydrolysis.